Systems described herein provide techniques for establishing and modifying user plane communications sessions between Long-Term Evolution (“LTE”) User Equipment (“UE”) devices, connected to LTE base stations, and a Fifth Generation (“5G”) core network. An LTE-5G Interworking function (“LTE-5G IWF”) may logically generate a virtual 5G UE and/or 5G base station, map a LTE UE to the virtual 5G UE, and cause the establishment of a Protocol Data Unit (“PDU”) Session, at the 5G core network, with the virtual 5G UE. The LTE-5G IWF may provide PDU Session information to the LTE UE and base station to facilitate the establishment of user plane communications (e.g., via a tunnel) between the LTE UE and the 5G core network. The LTE-5G IWF may also receive modification parameters, such as Quality of Service (“QoS”) parameters, and provide instructions to the 5G core and/or to the LTE UE to handle traffic according to such parameters.